[LLVMdev] Use perf tool for more accurate time measuring on Linux

[Tobias Grosser]

On 20/05/2014 18:20, Yi Kong wrote:
> On 20 May 2014 16:40, Tobias Grosser <tobias at grosser.es> wrote:
>> On 20/05/2014 16:01, Yi Kong wrote:
>>>
>>> I've set up a public LNT server to show the result of perf stat. There
>>> is a huge improvement compared with timeit tool.
>>> http://parkas16.inria.fr:8000/
>>
>>
>> Hi Yi Kong,
>>
>> thanks for testing these changes.
>>
>>
>>> Patch is updated to pin the process to a single core, the readings are
>>> even more accurate. It's hard coded to run everything on core 0, so
>>> don't run parallel testing with it for now. The tool now depends on
>>> Linux perf and schedtool.
>>
>>
>> I think this sounds like a very good direction.
>>
>> How did you evaluate the improvements exactly? The following run shows e.g
>> two execution time changes:
>
> I sent a screenshot of original results in the previous mail. We used
> to have lots of noise readings, both from small machine background
> noise and large noise from the timing tool. Now noise from timing tool
> is eliminated and only few machine background noise is left. This
> makes manual investigation possible.

I think we need to get this down to zero even at the cost of missing 
regressions. We have many commits and runs per day, having one or two 
noisy results per run means people will still not look at performance 
changes.

>> http://parkas16.inria.fr:8000/db_default/v4/nts/9
>>
>> Are they expected? If I change e.g. the aggregation function to median
>> they disappear. Similarly the graph for one of them does not suggest an
>> actual performance change:
>
> Yes, some false positives due to machine noise is expected. Median is
> more tolerant to machine noise, therefore they disappear.

Right.

What I find interesting is that this change filters several results that 
seem to not be filtered out by our statistical test. Is this right?

In the optimal case, we should be able to set the confidence level we 
require high enough to filter out these results as well. Is this right?

Is there currently anything that blocks us from increasing the 
confidence level further reducing the noise level at the cost of some 
missed regressions?

> As suggested by Chandler, we should also lock the CPU frequency to
> further reduce machine noise.

I set it to 2667.000 Mhz on parkas16. You can try if this improves 
something.

Cheers,
Tobias